1. Field of the Invention
Multi-stage separation by crystallisation can be carried out continuously in a crystallisation column through which crystals are passed in counter-current with a liquid which is either a melt or a solution, with which they are in continuous exchange.
The process proceeds similarly to rectifying distillation. Crystals are formed at one end section of the column in a crystallisation zone. These crystals are transported in counter-current with the melt of the solution respectively, in the centre part, the exchange zone, which is the crystallisation column proper. At the other end section, in the melt or solution zone respectively, the crystals are converted into melt or else redissolved to a saturated solution by the application of heat.
If crystallisation is effected out of a mother liquor, the end of the crystallisation zone will furthermore be connected to a reflux cooler in which the mother liquor is condensed and then transported to the solution zone of the column. If the column is arranged vertically, the crystals are transported by gravity. This means that the crystals are transported downwards if they are heavier than the surrounding liquid. If the crystals are lighter than the liquid, they are transported upwards. In the first case the crystallisation zone is at the top of the column and the melt zone at the bottom. In the second case (ice-water) the crystalliser is at the bottom and the melt zone at the top of the column.
In the exchange zone, which is preferably kept in an adiabatic condition, a concentration gradient may establish itself owing to the recrystallisation process and/or an extractive washing process.
As is the case with continuous distillation, it is possible to draw off at both ends of the column. The mixture to be separated can be fed into the column at a suitable place. The amount drawn off is determined, inter alia, by the phase diagram of the components involved in separation and the mass-balance in the column. It is, of course, important to make the separation as economically as possible.
The application of counter-current crystallisation is curbed by the lack of a crystallisation column in which crystals can be brought simply and reliably into counter-current and effective contact with liquid. It is not surprising, therefore, that a number of attempts have been made to develop a crystallisation column in which purification and/or concentration processes of organic and inorganic compounds can be effected efficiently and economically on an industrial scale by means of crystallisation.
2. Description of the Prior Art
A number of developments are reviewed in the book Fractional Solidification by M. Zief and W. R. Wilcox (published by Marcel Dekker Inc. New York in 1967).
Prior art examples of crystallisation columns are those in which the transportation of crystals is regulated by means of a piston or a rotating helix.
With the piston principle, it is unavoidable that moving the piston downwards causes "wads" of crystals to form which uncontrollably form channels for the liquid phase. Furthermore, the mechanical system itself results in undesirable channel forming. Owing to this, the separating effect per unit of length of such columns is slight. Moreover, such columns do not lend themselves for central feeding, but only for end feeding.
With another prior art type of column, transportation is effected by a helix rotating in an annular gap. In a number of cases, good results have been obtained with this column. The method proves, however, to depend very much on the mixture to be purified. Even with minor variations in process conditions, a small separating effect or column breakage or other mechanical breakdowns can occur.
The workmanship of the helix has to be of a very high standard. Partly because of this, industrial application of this transport mechanism involves many complications.
The invention relates to a crystallisation column for countercurrent crystallisation, having a plurality of perforated plates positioned at regularly spaced intervals.
Such a column is known in the art.
With this prior art apparatus a vertically reciprocating pulsation at a frequency of 5-500/minute and a stroke preferably of 200 mm (with a column length of 1 meter) is transmitted to the liquid by application of a piston mechanism installed in a side tube. A drawback of this simple column is that the process variables, such as crystal size, mixing, and density of packing, cannot be adequately controlled. Moreover, this column does not provide enough possibilities of preventing crystal masses adhering to the wall or of breaking up large crystal agglomerates.
Since, with this prior art apparatus, vigorous mixing will occur owing to the considerable vertically reciprocating movement of the liquid in the compartments between the perforated plates, the separating action of this known column per unit of length (the plate number) is only low.